fix(stack/proxy): ensure wsProxy and httpProxy have correct type

Both, `httpProxy` and `wsProxy` inside `Proxymanager` have been assigned
the value of `new Proxy(...).serve()` which is an instance of `express`.

This was wrong and causes runtime errors when proxies are being stopped as
they don't have a `stop()` API.

packages/stack/proxy/src/index.ts

@@ -140,23 +140,20 @@ export default class ProxyManager {
       isWs: false,
       logger: this.logger,
       plugins: this.plugins
-    })
-      .serve(
-        this.host,
-        this.rpcPort,
-      );
+    });
+
+    this.httpProxy.serve(this.host, this.rpcPort);
     this.logger.info(`HTTP Proxy for node endpoint ${endpoint} listening on ${buildUrl("http", this.host, this.rpcPort, "rpc")}`);
+
     if (this.isWs) {
       this.wsProxy = await new Proxy({
         events: this.events,
         isWs: true,
         logger: this.logger,
         plugins: this.plugins
-      })
-        .serve(
-          this.host,
-          this.wsPort,
-        );
+      });
+
+      this.wsProxy.serve(this.host, this.wsPort);
       this.logger.info(`WS Proxy for node endpoint ${endpoint} listening on ${buildUrl("ws", this.host, this.wsPort, "ws")}`);
     }
   }